1. Field of the Invention
The present invention relates to an organic electroluminescent display device, and more particularly, to a dual panel-type organic electroluminescent display device and a method for manufacturing the same.
2. Discussion of the Related Art
Among flat panel displays, liquid crystal display (LCD) devices have been commonly used due to their thin profile, light weight, and low power consumption. However, the LCD devices are not self-luminescent and suffer from low brightness, low contrast ratios, narrow viewing angles, and large overall sizes.
Organic electroluminescent display (OELD) devices have wide viewing angles and excellent contrast ratios because of their self-luminescence. In addition, since the OELD devices do not require additional light sources, such as a backlight, the OELD devices have relatively small sizes, are light weight, and have low power consumption, as compared the LCD devices. Furthermore, the OELD devices can be driven by low voltage direct current (DC) and have short microsecond response times. Since the OELD devices are solid state devices, the OELD devices sufficiently withstand external impact and have greater operational temperature ranges. In addition, the OELD devices may be manufactured at low cost since only deposition and encapsulation apparatus are necessary for manufacturing the OELD devices, thereby simplifying manufacturing processes.
The OELD devices are commonly categorized as top emission-type and bottom emission-type according to a direction of the emitted light. Furthermore, the OELD devices may be categorized as passive matrix-type OELD devices or active matrix-type OELD devices depending upon methods of driving the devices. The passive matrix-type OELD devices are commonly used because of their simplicity and ease of fabrication. However, the passive matrix-type OELD devices have scanning lines and signal lines that perpendicularly cross each other in a matrix configuration. Since a scanning voltage is sequentially supplied to the scanning lines to operate each pixel, an instantaneous brightness of each pixel during a selection period should reach a value resulting from multiplying an average brightness by the number of the scanning lines to obtain a required average brightness. Accordingly, as the number of the scanning lines increases, the applied voltage and current also increase. Thus, the passive matrix-type OELD devices are not adequate for high resolution display and large-sized displays since the devices easily deteriorate during use, and power consumption is high.
Since the passive matrix-type OELD devices have many disadvantages with regard to image resolution, power consumption, and operational lifetime, active matrix-type OELD devices have been developed to produce high image resolution in large area displays. In the active matrix-type OELD devices, thin film transistors (TFTs) are disposed at each sub-pixel to function as a switching element to turn each sub-pixel ON and OFF. Accordingly, a first electrode connected to the TFT is turned ON/OFF by the sub-pixel, and a second electrode facing the first electrode functions as a common electrode. In addition, a voltage supplied to the pixel is stored in a storage capacitor, thereby maintaining the voltage and driving the device until a voltage of next frame is supplied, regardless of the number of the scanning lines. As a result, since an equivalent brightness is obtained with a low applied current, an active matrix-type OELD device has low power consumption and high image resolution over a large area.
FIG. 1 is a schematic circuit diagram of a pixel structure for an active matrix-type OELD device according to the related art. In FIG. 1, a gate line GL is arranged along a first direction, and a data line DL and a power supply line CSL that are spaced apart from each other are arranged along a second direction perpendicular to the first direction. The data line DL and the power supply line CSL cross the gate line GL, thereby defining a sub-pixel area SP. A switching thin film transistor (TFT) SwT, i.e., an addressing element, is formed at a crossing of the gate line GL and the data line DL, and a storage capacitor CST is connected to the switching TFT SwT and the power supply line CSL. A driving thin film transistor (TFT) DrT, i.e., a current source element, is connected to the storage capacitor CST and the power supply line CSL, and an organic electroluminescent (EL) diode E is connected to the driving TFT DrT.
When a forward current is supplied to the organic EL diode E, an electron and a hole are recombined to generate an electron-hole pair through the P(positive)-N(negative) junction between an anode, which provides the hole, and a cathode, which provides the electron. Since the electron-hole pair has an energy that is lower than the separated electron and hole, an energy difference exists between the recombination and the separated electron-hole pair, whereby light is emitted due to the energy difference.
FIG. 2 is a cross sectional view of a bottom emission-type organic electro-luminescent display (OELD) device according to the related art. In FIG. 2, first and second substrates 10 and 30 are spaced apart and are bonded together by a seal pattern 40. Each pixel P includes red, green, and blue sub-pixels SbP. A driving thin film transistor (TFT) DrT is formed at each sub-pixel SbP on an inner surface of the first substrate 10, and a first electrode 12 constituting an organic electroluminescent diode is connected to the TFT T. An organic electroluminescent layer 14 includes luminescent materials of red, green, and blue and is formed on the driving TFT DrT. In addition, a second electrode 16 is formed on the organic electroluminescent layer 14, whereby the first and second electrodes 12 and 16 induce an electric field to the organic electroluminescent layer 14.
A desiccant (not shown) is formed in an inner surface of the second substrate 30 to shield an internal portion of the OELD device from external moisture. The desiccant is attached to the second substrate 30 by an adhesive (not shown), such as semi-transparent tape.
In the bottom emission-type OELD device, for example, the first electrode 12 functions as an anode and is made of a transparent conductive material, and the second electrode 16 functions as a cathode and is made of a metallic material of low work function. Accordingly, the organic electroluminescent layer 14 is composed of a hole injection layer 14a, a hole transporting layer 14b, an emission layer 14c, and an electron transporting layer 14d formed over the first electrode 12. The emission layer 14c has a structure where emissive materials of red, green, and blue are alternately disposed at respective sub-pixels SbP.
FIG. 3 is a cross sectional view of a sub-pixel region of a bottom emission-type organic electroluminescent display device according to the related art. In FIG. 3, a thin film transistor area TrA, an emission area EmA and a storage capacitor area StgA are defined on a substrate 10. In the thin film transistor area TrA, a semiconductor layer 62, a gate insulating layer 63, a gate electrode 68, and source and drain electrodes 80 and 82 are sequentially formed. A storage capacitor and an organic electroluminescent (EL) diode E are connected to the source and drain electrodes 80 and 82, respectively. The storage capacitor is disposed in the storage capacitor area StgA, and the organic EL diode E is disposed in the emission area EmA. The storage capacitor includes a power electrode 72 and a capacitor electrode 64 that face each other with an insulating layer interposed between the power electrode 72 and the capacitor electrode 64. The capacitor electrode 64 is made of the same material as the semiconductor layer 62. The power electrode 72 extends from a power supply line (not shown). The thin film transistor and the storage capacitor are commonly referred to as array elements A. The organic EL diode E includes first and second electrodes 12 and 16 that face each other with an organic EL layer 14 interposed therebetween. The source electrode 80 of the thin film transistor is connected to the power electrode 72 of the storage capacitor, and the drain electrode 82 of the thin film transistor is connected to the first electrode 12 of the organic EL diode E. In addition, the array elements A and the organic EL diode E are formed on the same substrate 10.
The OELD device is fabricated through encapsulating the first substrate 10 including the array elements A and the organic EL diode E with the second substrate 30. In addition, a yield of the active matrix OELD device depends on individual yields of the thin film transistor and the organic EL diode E. Although the thin film transistor may adequately function, the yield of the active matrix OELD device varies due to the incorporation of impurities during the process of forming the organic layer. Accordingly, the yield of the active matrix OELD is reduced because of the impurities, and results in a loss of manufacturing costs and source materials.
In addition, the active matrix OELD device is a bottom emission-type device having high stability and variable degrees of freedom during the fabrication process, but has a reduced aperture ratio. Thus, the bottom emission-type active matrix OELD device is problematic in implementation as a high aperture device.
To overcome theses problems, a top emission-type active matrix OELD device, in which the array elements and the organic EL diode are formed on different substrates, has been suggested and developed.
FIG. 4 is a cross sectional view of a top emission-type active matrix OELD device of the related art, and FIG. 5 is an enlarged view of the region L of FIG. 4.
In FIGS. 4 and 5, first and second substrates 110 and 130 are spaced apart from each other and are bonded together by a seal pattern 140. Each pixel P includes three sub-pixels SbP. An array element 120 is formed at each sub-pixel SbP on the first substrate 110. The array element 120 formed at each sub-pixel sbP includes a driving thin film transistor (TFT) DrT and a connection electrode 112 connected to the driving TFT DrT. An organic EL diode E is formed on an inner surface of the second substrate 130 to correspond to each sub-pixel SbP. The organic EL diode E includes a first electrode 132 functioning as a common electrode, an organic EL layer 134 on the first electrode 132, and a second electrode 136 independently formed at each sub-pixel SbP. Additionally, a connection pattern 114 is formed between the second electrode 136 on the second substrate 130 and the connection electrode 112 on the first substrate 110 to electrically connect the second electrode 136 with the array element 120.
In the top emission-type OELD device where the organic EL diode E and the array element 120 are formed on different substrates, a process for attaching the first and second substrates 110 and 130 is performed under vacuum of about 10−2 Torr to about 10−3 Torr. The connection pattern 114 holds up the connection electrode 112 on the first substrate 110 and the second electrode 136 on the second substrate 130 under the above-mentioned vacuum condition. The connection pattern 114 has a contact area of about 10 μm2 to about 20 μm2. At this time, the inner pressure is concentrated on a local portion contacting the connection pattern 114, and the inner pressure is transmitted to the organic EL layer 134 through the second electrode 136.
Meanwhile, the second electrode 136 is commonly formed by a thermal evaporation method, and the second electrode 136 may have a very rough surface and a low film density. Thus, the second electrode 136 may be deformed or damaged due to the connection pattern 114 contacting the surface of the second electrode 136, more particularly, the pressure from the connection pattern 114.
Moreover, the deformation of the second electrode 136 causes transformation of the organic EL layer 134. The physical transformation of the organic EL layer 134 brings about stress on a surface of the organic EL layer 134 and changes in molecular structures. Therefore, the organic EL layer 134 may be degraded and characteristics of the organic EL layer 134 may be lowered.